This invention relates to magnetic field shimming systems. More particularly, the invention relates to magnetic field shimming systems for magnetic resonance imaging (MRI) magnet assemblies.
MRI magnet assemblies require an air gap large enough to receive all or part of an object to be scanned, and typically require an intensive, uniform magnetic field exhibiting an accuracy on the order of 10 parts per million (ppm) in a specified imaging volume within the air gap.
One known configuration of an MRI magnet assembly is known as open geometry MRI. Typically, open geometry MRI comprises opposing magnetic field generating assemblies connected to each other by one or more yokes or support members. Another known configuration is known as cylindrical geometry MRI, in which magnetic field generating assemblies and other magnetic field shaping components are arranged around a cylindrical structure.
The magnetic field in an MRI magnet assembly is typically created using permanent magnets, electromagnets, or a combination of the two. For example, one known MRI magnet assembly has a structure in which magnetic pole pieces are fixed facing each other at one end of each of a pair of permanent magnet structures in which an R—Fe—B-based magnet is used as the magnetic field generating element, the other ends are coupled by a yoke, and a static magnetic field is generated within the air gap between the magnetic pole pieces. Another known MRI magnet assembly has electromagnetic coils (including resistive coils, superconducting coils, and the like) wound around air or iron cores for the magnetic field generating element in place of the above-mentioned permanent magnet structures.
The uniformity of the magnetic field is greatly influenced by many factors. These factors include: the properties of the materials used in constructing the MRI magnet, the accuracy of manufacturing the MRI magnet, and the environment in which the MRI magnet is placed. Often times, variations in these and other factors result in an inhomogeneous field, ranging from several hundred ppm to several thousand ppm. To obtain the required uniformity for useful imaging, this field inhomogeneity must be corrected.
One method of correcting field inhomogeneity is to apply shims constructed of magnetic (e.g., ferrous) material and/or permanent magnetic material to opposing surfaces formed on the magnetic field generating assemblies. One example of this arrangement is described in U.S. Pat. No. 5,229,723 to Sakurai et. al. Sakural et. al describe an MRI-dedicated magnetic field generating device for generating magnetic fields within an air gap, including: a pair of permanent magnet assemblies opposite to each other to form an air gap therebetween; yokes for magnetically linking the permanent magnet assemblies; and magnetic pole pieces fixed to air-gap-confronting surfaces thereof. Based on this construction, intensities of the magnetic fields are increased by disposing a plurality of magnetic material segments on the same circle or a concentric circle on the opposing surfaces of the pole pieces or decreased by disposing a plurality of permanent magnet segments having a magnetizing direction opposite to that of the permanent magnet assemblies on the same circle or the concentric circle on the opposing surfaces of the pole pieces; or alternatively both the magnetic material segments and the permanent magnet segments are disposed on the same circle or the concentric circle.
There are, however, several drawbacks to this shimming arrangement. For example, a degree of care and effort must be used during the shimming process to stabilize the shims as they are being placed near the pole face, where they are subjected to the large magnetic forces of the permanent magnet assembly. Such care and effort inherently increases the time required to complete the shimming process.